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IC 8912 



Bureau of Mines Information Circular/1983 




-21983 



Design Criteria for Rapid- Response 
Pneumatic Monitoring Systems 



By Charles D. Litton 




UNITED STATES DEPARTMENT OF THE INTERIOR 



it 



Information Circular 8912 

Design Criteria for Rapid-Response 
Pneumatic Monitoring Systems 

By Charles D. Litton 




UNITED STATES DEPARTMENT OF THE INTERIOR 
James G. Watt, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 



Ah 



This publication has been cataloged as follows: 



Litton, C. D. (Charles D.) 

Design criteria for rapid-response pneumatic monitoring systems. 

(Information circular/ U.S. Bureau of Mines ; 8912) 

Includes bibliographical references. 

Supt. of Docs, no.: I 28.27:8912. 

1. Mine ventilation— Equipment and supplies. 2. Mine safety- 
Equipment and supplies. 3. Pneumatic control. I. Title. II. Series: 
Information circular (United States. Bureau of Mines) ; 8912. 



TN295.U4 [TN301] 622s [622'. 42] 82-600269 



CONTENTS 



Page 



Abstract 1 

Introduction 2 

General description 3 

The tubing system > 3 

The control station 5 

Time considerations • 7 

Contaminant transport time , i t 8 

Tube traveltime , t£ 8 

System sequence time, t S eq * H 

Design examples 12 

Design example I 12 

Design example II 14 

Pneumatic monitoring for submicrometer particles 17 

Summary and discussion 19 

Conclusions 21 

Appendix. — List of symbols 22 

ILLUSTRATIONS 

1. Schematic showing appropriate locations for auxiliary components along the 

length of a sampling tube 5 

2. Generalized schematic of a pneumatic monitoring system 6 

3. Block, diagram showing the electronic functions required for a pneumatic 

monitoring system 6 

4. Characteristic flows for three small vacuum pumps as a function of the 

pressure drop across the pump 9 

5. Maximum recommended tube lengths as a function of tube inside diameter for 

use in pneumatic monitoring systems 10 

6. Schematic showing the relative locations of the sampling tubes and control 

station for the belt entry discussed in Design Example I 15 

7. Schematic showing the relative locations of the sampling tubes and control 

station for the three returns discussed in Design Example II... 18 

8. Maximum tube lengths as a function of inside tube diameter for pneumatic 

monitoring systems designed to detect submicrometer particles 19 



DESIGN CRITERIA FOR RAPID-RESPONSE PNEUMATIC MONITORING SYSTEMS 

By Charles D. Litton } 



ABSTRACT 

This Bureau of Mines report presents a discussion of the essential 
components of pneumatic monitoring systems and their associated func- 
tions. Design criteria are presented which can be used for the design 
and fabrication of pneumatic monitoring systems having total system re- 
sponse times on the order of 15 to 30 min. To illustrate the utility of 
these design criteria, two detailed design examples are presented. 

■Supervisory physical scientist/ Pittsburgh Research Center, Bureau of Mines, 
Pittsburgh, Pa. 



INTRODUCTION 



Improving the degree of safety afforded 
underground mine personnel is a major 
goal of the Bureau of Mines' research 
program. This report discusses in detail 
a methodology for monitoring of mine air 
contaminants that has the potential to 
improve underground mine safety by pro- 
viding early warning of developing haz- 
ards. The data on which this report is 
based was acquired from both in-house re- 
search projects and contractual efforts 
originating from the Fires and Explosions 
Group of the Pittsburgh Research Center, 
Bureau of Mines. 

Many potential hazards in underground 
mines are preceded by, or result in, the 
formation of contaminants that are car- 
ried throughout the mine by the imposed 
ventilation. Continuous monitoring of 
the mine air for contaminants has the po- 
tential to provide early warning of asso- 
ciated hazards in a time sufficient to 
successfully initiate control measures 
and to ensure the safety of underground 
personnel. 

Two techniques exist for continuous 
monitoring of the mine air. The first 
technique (called the electronic method) 
consists of placing one or more contam- 
inant sensors (called sensor packages) at 
carefully chosen locations within an un- 
derground mine. These sensor packages 
are then hard-wired to a remote station 
that provides electrical power for the 
sensors and accepts electrical signals 
from the sensors. The remote station may 
also contain a multiplexing function by 
which sensor signals are transmitted via 
two-conductor communication lines to a 
central, master control center. Depend- 
ing upon the size of the mine and the 
number of sensor packages, this method 
can be relatively simple (for instance, 
one sensor package with a remote station 
that also serves as the master control 
center); or it can be complex (for in- 
stance, several sensor packages with sev- 
eral remote stations controlled by a 



master control center). For this tech- 
nique, every underground monitoring point 
has a sensor package containing one or 
more sensors. 

In the second technique (called the 
pneumatic method), sensor packages are 
replaced by tubes. For every underground 
monitoring location, a tube extends from 
that location to a central control sta- 
tion. At this control station, pumps 
continuously pull samples of mine air 
from the monitoring locations through the 
tubes and then exhaust the samples into 
one or more contaminant sensors. Sensor 
outputs can be recorded and displayed at 
this station, or the outputs can be mul- 
tiplexed to a master control center for 
recording and display. 

The pneumatic method has been used suc- 
cessfully for continuous monitoring ap- 
plications where the rate of development 
of a particular hazard is slow with re- 
spect to the overall response time of the 
system. Such applications would include 
the continuous monitoring for detection 
of spontaneous combustion and for assess- 
ing the status of underground sealed fire 
areas. 2 It has been argued, and quite 
erroneously, that imposed time delays due 
to tube transit times and sequencing 
times are unacceptable for applications 
where the rate of development of a par- 
ticular hazard can be quite rapid. 

2 Burgess, D., and H. Hayden. A Carbon 
Monoxide Index Monitoring System in an 
Underground Coal Mine. Soc. Min. Eng. , 
AIME, Ann. Fall Meeting, Salt Lake City, 
Utah, September 1975, SME Preprint 75- 
F-350, 25 pp. 

Chamberlain, E. A. C. , and D. A. Hall. 
The Practical Early Detection of Spon- 
taneous Combustion. Colliery Guardian, 
London, May 1973, pp. 190-194. 

Dalverny, L. E. , Z. J. Fink, and J. P. 
Weinheimer. Continuous Gas Monitoring 
Using Tube Bundles at the Joanne Mine 
Fire. BuMines TPR 92, 1975, 12 pp. 



While it is true that these times can 
be significant, it is also true that they 
can be dealt with quantitatively so that 
their effects can be minimized. Further, 
by presenting design criteria, primarily 
with respect to these time constraints, 
it becomes possible to calculate, in 
advance, the overall time response of a 
pneumatic monitoring system. And this 
information can subsequently be used to 
determine if a pneumatic monitoring sys- 
tem can be designed to meet the mon- 
itoring requirements for a proposed 
application. 



It is the intent of this report to 
describe the components necessary for 
fabrication of a pneumatic monitoring 
system and to present, in detail, design 
criteria that can be used to assess the 
limiting time responses of such systems. 
In so doing, it becomes apparent that, 
through prudent engineering design, pneu- 
matic monitoring systems can be used to 
monitor for hazards that develop quite 
rapidly. 



GENERAL DESCRIPTION 



A pneumatic monitoring system can be 
considered to be composed of two major 
elements. First, there is a set of two 
or more tubes extending from a central 
control station to the monitoring loca- 
tions. These tubes are used to convey 
air samples from the monitoring locations 
to the control station where various con- 
taminant levels are determined. Second, 
there is the central control station 
where pumps continuously purge the sam- 
pling tubes and present the air samples 
to the contaminant sensors for measure- 
ment. These two elements, and their com- 
ponents, will now be discussed in some 
detail. 

THE TUBING SYSTEM 

The set of tubes used for conveying air 
samples is commonly in the form of a 
bundle of tubes (hence, the name "tube 
bundles"), although for rapid-response 
systems, single tubing of >0.635-cm 
ID(> 1/4-in-ID) may often be required. 
Commercially available tube bundles con- 
sist of two or more tubes per bundle 
with each tube within a bundle having 
either an 0.427-cm ID(~l/6-in-ID) or an 
0.635-cm ID (1/4-in-ID). The number of 
tubes per bundle ranges from 2 to 37 for 
the smaller size tubing, and from 2 to 19 
for the larger size. Individual tubes 
are usually of polyethylene, although 
metal tubing may also be available from 
some manufacturers. Each bundle is cov- 
ered by an outer sheath of polyethylene 



(~l/8- to 1/4-in thick), which serves to 
protect the enclosed tubing. Bundles are 
generally in the form of reels, resem- 
bling reels of multiconductor electrical 
cable. Single tubing is also usually 
made of polyethylene; other tubing mate- 
rials are generally available, but at 
somewhat higher cost. 

Any single tube forms a path of com- 
munication between the monitoring lo- 
cation and the control station. Air 
samples must flow through a tube un- 
altered so that a contaminant measurement 
is truly indicative of the actual lev- 
els at the monitoring location. This 
constraint implies that some consider- 
ation should be given to the potential 
reactivity of contaminants during tube 
transport, either with the tube or with 
other gases in the air sample. In gen- 
eral, contaminants of interest, such as 
methane, CO, C0 2 , or smoke particles, 
are nonreactive and can be safely 
transported through polyethylene tubing. 
On the other hand, sulfur-containing 
contaminants, such as H 2 S or S0 2 , 
tend to react with either water vapor 
or condensed water during transport. 
Oxides of nitrogen can react with the 
polyethylene; if these gases are 
to be monitored, tubing made of non- 
reactive materials would be recom- 
mended. The more stable gases, nitrogen 
and oxygen, are nonreactive and can be 
safely transported through polyethylene 
tubing. 



In addition to the tubing, certain 
other components are generally required 
or recommended. At the sampling loca- 
tion, end-of-line dust filters should 
usually be attached to the sampling tubes 
to prevent accumulation of dusts within 
the tubes; this can sometimes result in 
clogging of the sample tubes. These 
filters are generally of large surface 
area and provide little resistance to the 
flow. 

If a tube bundle is used containing 
four or more tubes, it is generally rec- 
ommended that connections between tube 
bundles be protected by a junction box. 
A junction box is simply a rectangular 
box, made of heavy-duty metal or plastic, 
that can be mounted on the ribs of an 
entry. The lid of the junction box is 
easily removed, and tube bundles, one 
coming into the box from either side, can 
be connected and the lid replaced. The 
tube bundles entering the junction box 
are held firmly in place by brackets so 
that no tension is exerted on the con- 
nectors themselves. For bundles contain- 
ing fewer than three tubes, or for single 
tubing, it is recommended that, where 
connections are to be made, the tubing be 
firmly secured to the rib or roof on 
either side of the connection so that no 
tension is exerted on the connection. 
Once the connection is made, it should be 
wrapped with heavy-duty tape. 

Depending upon the location of the mon- 
itoring points relative to the control 
station, temperature differences along 
the path of a sampling tube may be en- 
countered; this can result in condensa- 
tion of water within the tubes. 

While water condensation within a tube 
generally will not affect the performance 



of the system, the transport of water 
into the contaminant sensors can signif- 
icantly affect their performance. For 
this reason, water traps are usually in- 
serted in the sampling line at the con- 
trol station just prior to the tube con- 
nection to the solenoid valves (use 
described below). 

In some applications, the gas sample 
may be flammable, and while no energy is 
available along the tube length to ignite 
such a flammable mixture, ignition could 
occur at the control station where power 
is supplied to the system. If ignition 
were to occur at this point, it is con- 
ceivable that flame propagation back 
through a sample tube could occur; this 
potentially could ignite a flammable gas 
mixture in the vicinity of the monitoring 
location. To avoid this potential prob- 
lem, simple in-line flame arrestors 
should be inserted in the sampling lines 
between the water traps and solenoid 
valves. It should be noted that no such 
occurrences have ever been observed, and 
these devices are recommended primarily 
as a precautionary measure. 

For applications where pneumatic moni- 
toring systems are to be designed for 
rapid response (see following sections), 
it is generally required that all sam- 
pling tubes have the same volumetric flow 
rate through them. For this reason, such 
systems should have manual valves in- 
serted in the sampling lines between the 
flame arrestor and the solenoid valves. 
These valves can then be used for obtain- 
ing the proper flow rate through each 
tube in the system. Such valves can be 
in the form of "needle" valves or "ball" 
valves; in most cases, the latter will 
suffice. 



If a sampling tube were fitted with 
all of these auxiliary components, then 
from the monitoring location up to the 
three-way solenoid valves, these com- 
ponents should be installed as per 
figure 1. Note that these components, 
except for dust filter and junction box, 
can be installed at the control 
station. 

THE CONTROL STATION 

The control station is literally the 
heart of a pneumatic monitoring system. 
It is at this station that pumps continu- 
ously purge all of the sampling tubes , 
with each tube sequentially connected in 
a regular, repeating fashion to the con- 
taminant sensors. A generalized, pneu- 
matic monitoring system is depicted in 
figure 2. 



Sampling tubes entering the control 
station are connected to the input ports 
of three-way solenoid valves (or their 
equivalent). Multiport rotary valves are 
available that can accomodate 12 to 24 
incoming tubes. For systems using 12 or 
more sampling tubes, these valves can be 
used in the place of individual solenoid 
valves and at a reduced cost. One output 
port of each solenoid valve is connected 
to a large scavenger pump, while the 
second output port is connected to a 
smaller, sample pump. Typically, the 
scavenger pump continuously purges all of 
the sampling tubes, except for one, which 
is connected through the valving to the 
sampling pump. Each solenoid valve is 
energized in a regular, repeating fashion 
by a combination timer-controller (see 
figure 3) so that the contaminant levels 
within each tube can be determined. 



End of line 
dust filter 








Junction 
box 







r 



Control station 



Water Flame Ball 
tra P arrester VQ lve 



3- way 
solenoid 
valve F | 0wt0 



I 



scavenger 
pump 



Flow to 

sample 

pump 



FIGURE 1. - Schematic showing appropriate locations for auxiliary components along the length of 
a sampling tube. Not all components are required for each system. 



Sample 
pump 




Tubes from 
monitoring locations 



.Solenoid valves 
or equivalent 



^Sensor Ij- 



(sensor 2j- 



( Sensor nj 



>- Exhaust 



Scavenger 
pump 



Exhaust 

FIGURE 2. - Generalized schematic of a pneu- 
matic monitoring systemshowing the essential air 
sampling components at the control station. 

Once a solenoid valve is energized, the 
flow from that sampling tube is diverted 
to the sample pump, which in turn pro- 
vides flow to the contaminant sensors, as 
shown in figure 2. The number and type 
of sensors required will depend upon the 
requirements of the monitoring system 
and the information it is intended to 
provide. It is worth noting that, for 
this type of system, many contaminants 
can be measured at a single convenient 
location if so desired. 



Again, care should be taken in order to 
avoid any degradation of the air sample 
by components within the system. Pumps 
with nonreactive internal components are 
usually available and are generally rec- 
ommended for this type of system. Simi- 
lar consideration should be given to any 
component that is in direct contact with 
the air sample. 

In order for the control station to 
operate properly, it must contain elec- 
tronic functions. These functions are 
depicted in block form in figure 3. The 
combination timer-controller is used to 
provide power to the individual solen- 
oids. In general, the timer is set so 
that a single tube is sampled for a cer- 
tain period of time. At the end of that 
time interval, the timer generates a 



Timer 
controller 



Solenoid 

valves 

or 

equivalent 



Sample tube 
identifier 



(Alarm ^N 
outputs/^ 



Signal 
processor 



_jf Data "\ 
v^recorder/ 



Sensor 
outputs 



Contaminant 
sensors 



FIGURE 3. = Block diagram showing the elec= 
tronic functions required for a pneumatic moni = 
toring system. 



pulse that is received by the controller. 
The controller in turn produces a relay 
closure for the next solenoid valve, and 
so on. The timer-controller unit also 
produces a second output, which indicates 
the current tube whose sample is being 
sent to the sensors. This output, along 
with the outputs from the sensors, is 
connected to a signal processor. 

The signal processor in turn provides 
for alarm outputs and also for input to 
some type of data recorder, such as a 
strip-chart recorder. If remote alarms 
and remote data recording are required, 
the signal processor can also contain 
multiplexing and telemetry functions. 
This unit can be very simple (such as 
providing for alarms when contaminant 
levels exceed some value) or it can be 
more complex (such as providing for data 
recording and multiplexing or telemetry 
functions). In general, the signal pro- 
cessor should indicate the tube number 
associated with the alarm or data output. 

If desired, the signal processor can be 
used to provide information on the opera- 
tional status of the system. It can con- 
tain electronics that, for instance, 



supervise the sensors and other elec- 
tronic functions and provide indication 
of system malfunction. Tube integrity 
can also be monitored by inserting 
an electronic flowmeter in the line 
connecting the solenoid valves to the 
sample pump. This device can in turn be 
connected to the signal processor. Since 
all sampling tubes are required to have 
the same approximate flow, a low-flow 
indicator would signal that the tube is 
being blocked while a high-flow indi- 
cator would signal that a tube has been 
broken. 

Again, the number of electronic compon- 
ents and their complexity will depend 
upon the system requirements. 

This section has discussed the opera- 
tion of a generalized pneumatic monitor- 
ing system and the components required to 
make such a system operational. In prin- 
ciple, this information is sufficient to 
fabricate a pneumatic monitoring system. 
However, in order to design a system with 
a known response time, it is important 
that the various parameters that deter- 
mine the overall system response time be 
discussed. 



TIME CONSIDERATIONS 



A continuous monitoring system is in- 
tended to provide certain specific in- 
formation that can be used to signal the 
development of a potential hazard. For a 
monitoring system to function properly, 
it must be capable of providing this 
information in a time sufficient to suc- 
cessfully initiate control measures and 
to ensure the safety of underground 
personnel. Consequently, a monitoring 
system designed to protect against some 
hazard must have a maximum response time 
that is less than the anticipated devel- 
opment time of that hazard. 



If the hazard development time is x m 
and the maximum system response time is 



s » 



then 



T s < T m 



(1) 



if the system is to achieve its intended 
purpose. When some contaminant is re- 
leased into the mine air, that contami- 
nant is carried downstream at the venti- 
lation velocity, v f . If the monitoring 
location is some distance, £, from the 
point of origin of the contaminant, then 
the time necessary for the contaminant to 



B 



reach the monitoring location, if » is 
given by 

x t ■ 4/v f (2) 

Now, for a pneumatic monitoring system, 
once the contaminant reaches the monitor- 
ing location, it must travel through a 
tube of some length, l Q , before it reach- 
es the contaminant sensor. Provided that 
the pumps have sufficient capacity, the 
traveltime, t^ , for laminar flow (Rey- 
nolds number < 1,800) through a tube 
of length, l Q , in meters, and inside 
diameter, d Q , in centimeters, is given 
by 



0.35 £ d . 



(3) 



Since all sampling tubes within the sys- 
tem should have the same volumetric flow 
rate, the maximum tube traveltime will 
occur for the longest sampling tube in 
the system. 

Once the contaminant reaches the cen- 
tral station, the maximum amount of time 
that it will take before the contaminant 
is detected will be one complete sequenc- 
ing time, t SE q. This results from the 
fact that this particular tube may have 
been sampled just prior to the contam- 
inant's reaching the central station, and 
should this happen, then the remaining 
tubes will be sampled before the 
contaminant-bearing tube 
again. If the sample time 
T SAMP> tnen tne total time 
through the tubes is 



is sampled 
per tube is 
to sequence 



and this time must satisfy equation 1 
when used to monitor for a hazard that 
has a development time, x m . These 
three times will now be considered in 
detail. 

CONTAMINANT TRANSPORT TIME, t t 

The transport time, t + , (eq. 2) of a 
contaminant from its point of origin to a 
sampling point located a distance, £, 
downstream depends upon the value of I 
and the ventilation velocity, Vf , within 
the entry. Clearly, if i is zero, then 
x + is zero and this time need not be con- 
sidered. (See Design Example II.) How- 
ever, in many applications, the point of 
origin of the contaminant may not be 
known precisely, and sampling locations 
may have to be distributed in some logi- 
cal fashion in order to protect an entry 
(or entries) from a potential hazard. 
If, for instance, an entry is to be pro- 
tected from a hazard (such as fire) , and 
the point of origin of the associated 
contaminants could be any point along the 
entry, the sampling points should be 
spaced at some regular interval along 
the entry. If the entry length is £ E , 
then for n tubes, the spacing between 
sampling tubes, i D> will be given 
by 



*D = * E / n , 



(6) 



and the maximum transport time would be 
one spacing divided by the ventilation 
velocity, or 



SEQ 



= n t 



SAMP 



(4) 



Tf 



- *E 
nvf 



(7) 



where n is the number of tubes in the 
system. 

The sum of these three times, (eqs. 2- 
4) is the maximum response time of the 
pneumatic monitoring system; that is, 



x s = A/v f + 0.35 Jl d + n r s 



AMP 



(5) 



TUBE TRAVELTIME, Tjl 

The tube traveltime, tjj, given by 

equation 3, depends not only upon the 

tube length and diameter but also upon 

the capacity of the pumps used to provide 

flow through # the tube. The volumetric 

flow rate, Q v , necessary to provide a 



traveltime, t^, is the tube volume di- 
vided by the tube traveltime; that is, 



1,000 



• lOOird* „ 
Qv = 7T- 2 - *o 



(8) 



with # £ in m, d Q in cm, t^ in seconds, 
and Q v in cm 3 /s. Substituting t^ from 
equation 3 yields 



Qv = 2 " d 



(9) 



Q corresponds to the volumetric flow for 
which the Reynolds number is ~1,800. 



Now, for a tube of length, l , 
side diameter, d Q , the required 
drop across that tube necessary 
vide a traveltime, t^ , can be 
be3 

I 2 1 
AP = P A -P+ = 0.076 ^7 — 
A * d o T l 



to pro- 
shown to 



(10) 



where 



P A = ambient 



and 



atmospheric pres- 
sure at the open end of 
the tube, assumed equal to 
760 mm Hg (1 atm) 

P + = pressure at the end of the 
tube just before entering 
the pumps (mm Hg) 



The capacity of the pumps used must 
be capable of providing the flow, 
Q v (eq. 9), at the pressure drop, 
AP (eq. 10). In general, the small 
vacuum pumps used in these systems have 
free air capacities ranging from 236 
cm 3 /s (0.5 ft 3 /min) to 7.1 x 10 3 cm 3 /s 
(15 ft 3 / min) , and are capable of contin- 
uous operation at pressure drops 
across the pump of 580 to 680 mm Hg. 
Such pumps show a linear decrease in 
flow rate with pressure drop, AP, 

^Hertzberg, M., and C. D. Litton. Mul- 
tipoint Detection of Products of Combus- 
tion With Tube Bundles. Transit Times, 
Transmissions of Submicrometer Particu- 
lates, and General Applicability. Bu- 
Mines RI 8171, 1976, 40 pp. 



800 



600- 



E 
u 

S* 

o 



.cf 400 *- 



200- 




200 400 

AP, mm Hg 



600 



FIGURE 4. = Characteristic flows for three 
small vacuum pumps as a function of the pres- 
sure drop across the pump. 

across the pump. Some typical pump 
curves are shown in figure 4, illus- 
trating this linear dependence on AP. 
These curves can be represented by the 
general expression 



Qv= Qo l- 



AP 
AP 



(ID 



Where Q = free air capacity (cm 3 /s) of 
the pump, and AP m = maximum pressure drop 
the pump can provide (mm Hg). Assuming 
the exhaust of the pump to be at atmos- 
pheric pressure, the pressure drop across 
the pump equals the pressure drop across 
the tube. By substituting the appropri- 



ate expressions for Q v , AP, and t 



: l> 



the 



required pump characteristics, Q and 



10 



AP, 



can be determined. The resulting 



expression is 



225 d Q 
1 - 0.22 i. 



(12) 



AP r 



V 



Equation 12 represents an explicit 
statement of the pump characteristics, Q 
(free air capacity), and AP m (maximum 
pressure drop), necessary for a tube of 
length i Qi in meters, and inside diam- 
eter, d Q , in centimeters, such that the 
traveltime, t^, can be expressed by equa- 
tion 3. Equation 12 should be used for 
determining the flow using the maximum 
tube length of a system, and is appli- 
cable only to flow through a single tube. 
Consequently, equation 12 should be used 
for determining the sample pump required 
for the system. 

Now, the scavenger pump must continu- 
ously purge all tubes except for one. 
Since it is required that the volumetric 
flow rate through all tubes be the same, 
then the capacity of the scavenger pump 
must satisfy 



Qscav > (n-1) Q 



(13) 



where n is the total number of tubes in 
the system. 

Equations 12 and 13 are important de- 
sign criteria for selection of pumps that 
are to be used in pneumatic monitoring 
systems for which the time response is 
crucial to the overall performance of the 
system. And, when pumps are used which 
satisfy these requirements, the tube 
traveltime of equation 3 can be used 
directly to determine the x^-component of 
the overall system response time. 



Equation 12 also provides a convenient 
means for determining the maximum length 
of a given tubing of inside diameter, d Q . 
As the denominator of equation 12 ap- 
proaches zero, the required pump capac- 
ity, Q , approaches infinity. By set- 
ting the denominator equal to zero, the 



maximum tube length as a function of in- 
side tube diameter can be determined as 



* MAX = 4.55 AP m d 3 



(14) 



In practice, it is recommended that the 
maximum tube length not exceed 90% of 
this value in order to minimize pump 
requirements. Then, the maximum recom- 
mended tube length is 



U MAX >REC < 4.1 AP m d Q 3 



(15) 



Figure 5 is a plot of (& 'REC versus 
d Q for an assumed AP m of 580 mm Hg. 

Using tube lengths in excess of their 
recommended lengths does not mean that 
flow through the tubes will cease. Flow 
will still occur, but at a much-reduced 
rate, resulting in longer tube travel- 
times. Further, when using these longer 




0.5 1.0 1.5 

INSIDE TUBE DIAMETER, d , cm 



FIGURE 5. - Maximum recommended tube 
lengths asa function of tube inside diameter 
for use in pneumatic monitoring systems. 



11 



lengths, it becomes difficult to deter- 
mine, in advance, what the values of 
these traveltimes will be, and equation 3 
is no longer valid for these cases. In 
general, then, tube lengths greater than 
their recommended lengths should be used 
primarily in applications where the time 
response of the system is not crucial to 
its performance. Such applications might 
include the monitoring for spontaneous 
combustion, or the monitoring of sealed 
areas within a mine. For this type of 
application, the tube traveltimes can be 
measured and the time response of the 
system can usually be determined, even 
though it may not be crucial to the over- 
all intended purpose of the system. 



inside tube diameter is known, then this 
purge time is 



T M = *y*m x 6 



4Q V 



(16) 



where the superscript "M" refers to the 
main exhaust line of the sample pump. 

Once the sample of gas reaches the 
"TEE" connector, it flows to the contam- 
inant sensor at a reduced rate, Q s , de- 
termined by the flow requirements of the 
sensor. This sensor purge time is given 
by 



■ S - 



Trd s 2 JL 
4Qs 



(17) 



SYSTEM SEQUENCE TIME, t seq 

For a pneumatic monitoring system com- 
posed of n tubes, the time to sequence 
from one tube through the remaining tubes 
and back to the original tube is given by 
equation 4. This time is determined by 
the number of tubes, n, and the sampling 
time per tube, tsamp* ^he sampling time 
per tube can be determined from a knowl- 
edge of the response times of the contam- 
inant sensors, and the time required to 
purge the tubing that connects the sample 
pump to the contaminant sensors. 

Generally, contaminant sensors are con- 
nected via a "TEE" connection to the ex- 
haust line of the sample pump (see fig- 
ure 2), with each sensor requiring a flow 
of 1 to 2 L/min. As a general rule, the 
purge time between samples from different 
tubes should be the time necessary to 
displace approximately 6 times the volume 
of the connecting tubing. The total flow 
rate through the exhaust of the # sample 
pump will equal the flow rate, Q v , pre- 
viously discussed. If the length of tub- 
ing, £ m , in centimeters, from the sample 
pump to the last "TEE" connector to the 
contaminant sensors is known, and the 



where the superscript "s" refers to the 
contaminant sensor tubing between the 
"TEE" connector and the sensor. 

In general, the sensor purge time is 
much greater than the purge time for the 
main sample exhaust so that in calculat- 
ing purge times equation 17 need only be 
considered. This purge time can be re- 
duced by using small lengths of tubing to 
connect the contaminant sensors to the 
sample pump exhaust, and by using smaller 
diameter tubing. Common 0.318-cm-OD 
(1/4-in-OD) tubing with a 0.43-cm-ID is 
recommended. 

Also, if the maximum response time, tr, 
of the contaminant sensors is known, then 
the interconnecting tubing should be of a 
length such that the additional purge 
time is less than about 20% to 25% of 
this response time. For instance, if the 
value of tr is 60 sec and the required 
sensor flow is 16.7 cm 3 /s (1.0 L/min), 
then for the above size tubing (0.43- 
cm-ID) , the maximum length of connecting 
tubing corresponding to 20% of 60 s is 
calculated from equation 17 to be 230 cm 
(~7.5 ft). Since, in general, sensors 
are located very close to the sample 



12 



pump, this condition can usually be sat- 
isfied with little effort. 

Then the total sample time is equal to 
the purge time plus the sensor response 
time; that is, 



T SAMP " T p + T R" 



(18) 



But, since x| must be less than 25% of 
t r , then the total sample time can be 



written in terms of 
response time: 



the maximum sensor 



T SAMP - 1>25 T R 



(19) 



Equation 19 is generally valid for 
sensors with response times greater 
than 30 s. If faster sensors are used, 
then equations 17 and 18 should be used 
to determine the sample time per 
tube. 



DESIGN EXAMPLES 



The preceding sections have discussed 
the components necessary for fabricating 
a pneumatic monitoring system and pre- 
sented guidelines for designing systems 
with rapid response times. This informa- 
tion can now be used for designing and 
fabricating pneumatic monitoring systems. 
In order to illustrate how this informa- 
tion can be used, the following two- exam- 
ples are given. These examples are in- 
tended to demonstrate the manner in which 
pneumatic monitoring systems can be de- 
signed to monitor for hazards that devel- 
op within a short period of time (~15 to 
30 min) . Clearly, there exist many other 
potential hazards which are slower to 
develop and for which the pneumatic moni- 
toring approach is a viable and cost- 
effective technique. 

DESIGN EXAMPLE I 

It is desired to use the intake air 
from a conveyor belt haulageway to pro- 
vide additional ventilation at the work- 
ing coal face. In order to use the belt 
air for face ventilation, a carbon mon- 
oxide (CO) monitoring system with an 
alarm threshold of 10 ppm above ambient 
is required along the belt haulageway. 
The entry cross section is ~9.8 m 2 (7- by 
15-ft) and is 1,800 m in length 
(~5,900 ft). The average ventilation 
velocity within the entry is 0.76 m/s 
(150 ft/min). Further, the minimum ac- 
ceptable alarm time for the CO monitoring 
system is 15 min (900 s). 



The objective, then, is to design a 
pneumatic CO monitoring system with a 
maximum response time of 15 min for a 
belt haulageway 1,800 m long with a ven- 
tilation flow of 0.76 m/s. 

To begin, a CO sensor must be chosen 
that is capable of responding at the 10- 
ppm-CO level and with a well-defined time 
response. Such a sensor is identified 
with a response time, t d , of 30 s. Then 



from equation 19, the 
tube is 



R» 
sample 



time per 



T SAMP 



= 37.5 s 



and the total sequence time, from equa- 
tion 4, will be 

T SEQ = 37 ' 5 n 

where the number of sampling tubes, n, is 
yet to be determined. 

The second step is to decide upon the 
location for the control station. One 
possibility is to locate the control sta- 
tion at the outby side of the belt entry. 
If this were to be done then the longest 
sampling tube would be approximately the 
length of the entry. From figure 5, it 
can be seen that the required tube di- 
ameter is ~0.9 cm (0.35 in) for this max- 
imum length of tubing. If these values 
for l Q (1,800 m) and d (0.9 cm) are in- 
serted into equation 3, then it is found 
that the tube travel time would be 567 s, 



13 



which leaves a total time of 333 s re- 
maining for contaminant transport and 
sequencing time; that is 

LL- + 37. 5n < 333. 
nv f 

If this equation is solved, using the 
values for £ E and v f , then it can be 
shown that no combination of n tubes can 
satisfy this time requirement. That is, 
any value of n will yield a total time 
greater than 333 s. Consequently, the 
control station must be located at some 
other point along the belt entry. 

For convenience, assume that the con- 
trol station can be located at the mid- 
point of the entry. Then the maximum 
tube length in the system will be 1/2 £ E 
or 900 m. From figure 5, the required 
tube diameter for this length is ~0.72 cm 
(0.28 in). This tube diameter is greater 
than 0.635 cm (1/4 in) but less than 
0.953 cm (3/8 in). The closest standard 
size tubing that can be used is 0.794 cm 
(5/16 in). Substituting these values 
into equation 3 yields a maximum tube 
travel time of 250 s, which leaves a 
total time of 650 s remaining for contam- 
inant transport and tube sequencing; that 
is, 

"lL_ + 37. 5n < 650 



Tube Travel Time 



*E 



nv 4 



Substituting the appropriate values for 
£ E and v f , and rearranging yields 

37.5n2-650n + 2,368 = 

Solving this equation for n indicates 
that when 5.2 < n < 12.1, the contaminant 
travel time plus the tube sequencing time 
will be less than 650 s. Consequently, 
the minimum number of sampling tubes 
which can be used is 6. Setting n=6, the 
following time components of the system 
result: 



Contaminant Transport Time 



Tp = 0.35 ^£_ d n = 250 s 

* nv f ° 



Tube Sequence Time 

T SEQ = 37. 5n = 225 s 
The maximum system response time is 

T S = T + + T^ + T SE Q = 870 S 

which is less than the required response 
time of 15 min (900 sec). 

Now, in order to select pumps to sat- 
isfy the tube travel time constraint, 
equation 12 indicates that, for a pump 
with a maximum AP m of 580 mm Hg (23 in 
Hg), its free air capacity must be 
greater than or equal to 562 cra 3 /s 
(~1.2 ft 3 /min); and equation 13 indicates 
that the free air capacity of the scav- 
enger pump must be > 2.81 x 10 3 cm 3 /s 
(~6.0 ft 3 /min). Since, in general, pumps 
with these capacities are readily avail- 
able, no problems are anticipated with 
regard to selection of pumps for this 
system. 

It should be noted that locating the 
control station at the midpoint of the 
haulageway does not represent the optimum 
location. The optimum location is that 
location which is central with respect to 
the number of sampling locations. In 
this case, the maximum tube length is 
given by 

W - 1/2 (*E - *£) - TJ *E 
and the total system response becomes 

^£- + 0.35 ( 5^ ^ JUd n + 37. 5n. 
nvf y 2n J t ° 



Tc = 



t+ = A£- = 395 s 



nv 



Now, to satisfy the time response re- 
quirement, n must be >3. That is, if n 
is <3, then the contaminant transport 
time plus the sequencing time exceeds 
900 s. Further, for n>4, the maximum 
tube length will be between 675 m and 



14 



900 m, and the closest standard size 
tubing that can be used for these lengths 
is 0.794 cm (5/16 in). For n=4, the sys- 
tem response time (from the above equa- 
tion) would be 930 s, which is too slow. 
However, when n=5, the system response 
time becomes 861 s, which is less than 
the required 900 s. Now, when n=5, the 
maximum tube length for a central control 
station is 720 m, and from equation 12, 
assuming AP m =580 mm Hg, the sample pump 
capacity required is 393 cm 3 /s (—0.84 
ft 3 /min); and for the scavenger pump, 
1.57 x 10 3 cm 3 /s (-3.4 ft 3 /min). 

Consequently, by locating the control 
station centrally with respect to the 
sampling tube locations, the number of 
tubes needed is five, rather than the 
necessary six if the control station were 
located at the midpoint of the belt 
entry. Also, this location would result 
in lower pump capacity requirements. 
While it is clear that the six-tube sys- 
tem would satisfy the time constraints 
for this application, the centrally lo- 
cated, five-tube system represents the 
optimum configuration. 

With the optimum five-tube system, 
there will be two tube lengths equal to 
720 m and 0.794-cm-ID tubing will be 
used. There will also be two tube 
lengths of 360 m each. Referring to fig- 
ure 5, it can be seen that for these two 
sampling tubes, 0.635-cm (1/4-in) ID tub- 
ing will suffice. The fifth sampling 
tube is located at, or very near, the 
control station, so that even smaller 
tubing could be used if desired. 

For this situation, the pneumatic CO 
monitoring system can be defined as 
follows: 



station. Flame arrestors in each sample 
line are also recommended. 

4. A local alarm shall be provided at 
the control station and provision made 
for a second remote alarm at the belt 
drive or other appropriate location. 

5. The sample pump used will require a 
capacity of -393 cm 3 /s (0.84 ft 3 /min); 
and the scavenger pump, a capacity of 
1.57 x 10 3 cm 3 /s (3.4 ft 3 /min). 

6. Five three-way solenoid valves with 
associated sequencing controls are 
required. 

7. A CO sensor with an alarm threshold 
of 10 ppm CO above ambient and a 30-sec 
response time will be used. 

8. If desired, data can be acquired on 
a continuous basis via a strip-chart 
recorder or some other type of data 
acquisition system. 

A conceptual layout of the sampling lo- 
cations and control station for this belt 
entry is shown in figure 6. 

Before concluding this example, it is 
worth noting that an electronic system 
could also be designed for this applica- 
tion. Such a system would require three 
individual CO sensors spaced at intervals 
of 600 m (1,970 ft), with each sensor 
hard-wired to a remote control station. 
Assuming each sensor to have a time re- 
sponse of 30 s , then the maximum response 
time for this system is the contaminant 
transport time plus the sensor response 
time, or 820 s. This system's response 
time is -40 s less than the response time 
of the pneumatic system. 



1. Five sampling tubes located at 
360-m intervals along the belt entry, and 
connected to 

2. A control station located 1,080 m 
(-3,540 ft) inby the belt drive. 

3. Each sampling tube will require an 
end-of-line dust filter at the sampling 
location and a water trap at the control 



DESIGN EXAMPLE II 

Return airways from three working coal 
faces meet at a common point and air 
flows outside via a single, common re- 
turn. Return airway 1 (Al) is 1,600 m 
(-1.0 mi) long with a ventilation veloc- 
ity of 0.64 m/s (125 ft/min); the second 
return (A2) is 2,600 m (~1.6 mi) long 
with a ventilation velocity of 1.02 m/s 






15 



r 



< f < S / / 



1,800 m 

|- 1.080 m 

i i— — 

l 



360 m A 



y , y s y y y y y / / s y / / / / / / / / / / / s s ^S/ /<<{<< /V/SSSSf/SS 



7 



s S s y ' y 



Monitoring 
locations 




\ 



)>))))> j ; > >> ) ; > > } > } > 1 1 ) / ??>>>> f >> > 



r-n 



"ft Roof 



Air flow 



Conveyor belt 



7 



i t > n ? > > f ? s >> r /n />>>>> > }? t r >>> t »>>>) ) tnttftuwuttt 



Control 
station 



FIGURE 6. - Schematic showing the relative locations of the sampling tubes and control sta= 
tion for the belt entry discussed in Design Example I. 



(200 ft/min); and the third return (A3) 
is 2,500 m (~1.55 mi) long with a venti- 
lation velocity of 0.89 m/s (175 ft/ 
min) . The length of the common return 
is 1,600 m (~1.0 mi) with a ventilation 
velocity of 2.55 m/s (500 ft/min). 

Under normal conditions, the methane 
content in each of the returns is less 
than 0.1%. However, the coal seam is 
beginning to dip, and it is feared that 
higher methane concentrations may be 
encountered; this would necessitate ven- 
tilation changes and could substantially 
increase the potential for methane-air 
explosions. Further, each return con- 
sists of extensive abandoned, mined-out 
areas, and there is legitimate concern 
about the tendency of spontaneous fires 
to develop within these areas. 

A continuous monitoring system is de- 
sired that is capable of providing an 
alarm should the methane content in a 
return exceed 0.5%, or should a spontane- 
ous fire occur. The system must be capa- 
ble of continuous monitoring for methane 
(CH 4 ) in the 0% to 1% range, with an 
alarm at methane levels >0.5%, and must 
also be capable of monitoring for methane 
in the 0% to 15% range once alarm has 
been given. Also, the maximum response 
time of the system at the 0.5% CH 4 level 
should be less than 30 min, so that ade- 
quate time is available to correct any 
problems that may arise. 

It has also been decided that a CO 
sensor with an alarm threshold of 15 ppm 



CO above ambient and a response time of 
30 sec is to be used for the detection of 
any spontaneous heatings that may occur. 
However, due to the long development 
times anticipated for the spontaneous 
heatings, the system response time could 
be much greater than 30 min and still be 
acceptable. Consequently, since both CO 
and CH 4 will be measured simultaneously, 
the system must be constrained to meet 
the 30-min alarm time for methane. 

The objective, then, is to design a 
pneumatic monitoring system that can sat- 
isfy all the above constraints. Since a 
CO sensor has already been identified, 
then an appropriate methane sensor must 
be selected. Such a sensor is identified 
with dual ranges of 0% to 1% and 0% to 
15%, and a maximum response time of 40 s. 
Also, the methane sensor automatically 
switches to the 0% to 15% range should an 
alarm at the 0.5% level occur. 

Now, since both CO and CH 4 are to be 
measured, six sampling tubes will be 
required. Three sampling tubes will be 
located just inby the three working faces 
and are to be used primarily for methane 
measurements, although CO will also be 
measured simultaneously. Three other 
sampling tubes will be located just inby 
the intersection of the three returns and 
will be used primarily for CO measure- 
ments, although methane will also be mea- 
sured simultaneously. Because the meth- 
ane sensor has a longer response time, 
the sampling time per tube will be lim- 
ited by its response. From equation 19, 



16 



the sampling time per tube will be 1.25 
x 40 s, or 50 s , and the total sequence 
time for six tubes (from equation 4) is 
6 x 50 s or 300 s; this is one component 
of the overall system response time. 

An intake entry parallels the common 
return, and it is decided to locate the 
control station within this entry, such 
that the distance from the common inter- 
section to the control station is 50 m 
(~165 ft). The Al return is the shortest 
return, and it can be seen from figure 5 
that tubing with a diameter greater than 
0.93 cm (0.37 in) would be required for 
this length (~1,650 m). The standard 
size tubing that meets this requirement 
is 0.953 cm (3/8 in) ID. For either 
entry, A2 or A3, the tubing size must be 
greater than about 1.03 cm (0.41 in). 
The standard size tubing that meets this 
requirement is 1.11 cm (7/16 in) ID. For 
the three primary CO monitoring tubes, 
their lengths are approximately 60 m 
(~197 ft), so that these three sampling 
tubes must have inside diameters >0.26 cm 
(>0.103 in). It is decided that standard 
tubing with a 0.635-cm ID (1/4 in ID) 
will be used for these three tubes. 

A determination of the location for the 
three primary methane sampling tubes must 
now be made. Since the tube sequencing 
time is fixed at 300 s, then the total 
time available for methane transport from 
the face to the sampling tube, plus the 
travel time through a tube is 1,500 s 
(25 min) . For any one of the three en- 
tries, the total methane transport time 
is equal to the distance of the sampling 
point from the face divided by the venti- 
lation velocity within that respective* 
entry. Also, the maximum tube length for 
an entry is equal to the total entry 
length plus the 50 m to the control sta- 
tion minus the distance from the face. 
If the distance from a face to the sam- 
pling tube location is denoted by £, in 
meters, then the respective times for 
methane transport and tube travel for 
each entry (eq. 2 + eq. 3) become 

For Al, 

t A] - 1.563 l + 0.334 (1,650-*) 



For A2, 

x A2 = 0.98 i + 0.389 (2,650-jO 
For A3, 

x A3 = 1.124 l + 0.389 (2,550-Jt) 

When i = 0, the sampling tubes are lo- 
cated essentially at the face; from the 
above equations, it can be seen that this 
situation corresponds to the lowest 
times. The reason for this is that the 
samples are traveling through the tube at 
a velocity much greater than the entry 
ventilation velocity, so that the result- 
ant travel time is much lower. When the 
distance of a sampling tube from the face 
begins to increase, the resultant times 
begin to increase. Since the maximum 
time available is 1,500 s, the maximum 
distances from the face in each entry can 
be determined by setting the respective 
times equal to 1,500 s and solving for I 
for each entry. The results are 

I (Al) < 772 m 

I (A2) < 794 m 

I (A3) < 691 m 

So long as the distances of the sam- 
pling tubes from the faces are less than 
the above respective values, the total 
response time of the system will be less 
than the required 30 min (1,800 s). It 
is decided to locate each sampling tube a 
distance of 50 m inby the face of each of 
the three returns. Then the maximum sys- 
tem response times, to methane, within 
each return are 

t s (Al) = 913 s (15.2 min) 

x s (A2) = 1,360 s (22.7 min) 

t s (A3) = 1,330 s (22.2 min) 

and the lengths of tubing required for 
each entry are 

£ (Al) = 1,600 m 



i (A2) = 2,600 m 



17 



i (A3) = 2,500 m 

The pump requirements for the system 
will be determined by the flow require- 
ments for the longest tube (2,600 m). 
From equation 12, assuming a AP m equal to 
580 mm Hg, the sample pump must have a 
capacity >896 cm 3 /s (>1.9 ft 3 /min). From 
equation 13, for this six-tube system, 
the capacity of the scavenger pump 
must be >4.48 x 10 3 cm 3 /s (>9.5 ft 3 /min). 
Since pumps with these capacities are 
generally available, no problems are an- 
ticipated with respect to pump selection. 

Then the pneumatic CH 4 -C0 monitoring 
system for this application is defined as 
follows: 

1 . Two sampling tubes will be placed 
in both returns A2 and A3 (four total) 
with one sampling tube of 1.11-cm (7/16- 
in) ID located 50 m (-164 ft) inby the 
face, and the second sampling tube of 
0.653-cm (1/4-in) ID located 10 m 
(~33 ft) inby the common intersection. 

2. In entry Al , one sampling tube of 
0.953-cm (3/8-in) ID will be located 50 m 
(~165 ft) inby the face and one sampling 
tube of 0.635-cm (1/4-in) ID will be lo- 
cated 10 m (~33 ft) inby the common 
intersection. 

3. All six sampling tubes will be con- 
nected to the control station located in 
an intake entry approximately 50 m 
(~164 ft) from the common intersection. 



7. A methane sensor with an alarm 
threshold of 0.5% and a response time of 
40 s will be used to measure the methane 
levels from each return. The methane 
sensor will automatically switch to a 0%- 
15% range should the 0.5% CH 4 level be 
reached. 

8. Local alarms shall be provided at 
the control station with provision made 
for identification of whether the alarm 
is for CO or CH 4 and also identification 
of the sampling location producing the 
alarm. Provision shall also be made for 
a second, remote alarm with the same 
identifying feature at another appropri- 
ate location. 

9. The sample pump used will require 
a capacity of ~896 cm 3 /s (1.9 ft 3 /min); 
and the scavenger pump, a capacity of 
4.48 x 10 3 cm 3 /s (9.5 ft 3 /min). 

10. If desired, provision can be made 
for continuous recording of CO and CH 4 
data, either at the control station or at 
the remote alarm location. 

A layout of the planned system is shown 
in figure 7. 

These two examples clearly indicate 
that pneumatic monitoring systems can be 
designed to warn of hazards that develop 
fairly rapidly. 

PNEUMATIC MONITORING FOR SUBMICROMETER 
PARTICLES 



4. Each sampling tube will require an 
end-of-line dust filter at the sampling 
location and both a water trap and flame 
arrestor at the control station in the 
sampling line just before entering that 
tube's three-way solenoid valve. 

5. Six three-way solenoid valves with 
associated sequencing controls are 
required. 



Submicrometer ("smoke") particles are 
one contaminant that can provide the ear- 
liest indication of developing fires. 4 
However, these particles, when trans- 
ported through tubes of a finite diam- 
eter, diffuse to the walls and hence can 
be lost completely within a sampling tube 
if certain precautions concerning tube 
diameter and tube travel time are not 
taken. For this reason, pneumatic 



6. A CO sensor with an alarm threshold 
of 15 ppm CO above ambient and a response 
time of 30 s will be used to measure the 
CO levels from each return. 



^Hertzberg, M. , C. D. Litton, and R. 
Garloff. Studies of Incipient Combustion 
and Its Detection. BuMines RI 8206, 
1977, 19 pp. 



IS 



Intake entry 



LEGEND 
2, 4 and 6 CH 4 sampling point 

/. 3 and 5 CO sampling point 



Common return 



Intake entry 




Return A3, 2,500 m 



Vx=0.89m/s 



Control 
station 



FIGURE 7. - Schematic showing the relative locations of the sampling tubes and control sta- 
tion for the three returns discussed in Design Example II. 



monitoring systems designed to measure 
the concentrations of submicrometer par- 
ticles have more stringent requirements 
on tube diameters and tube traveltimes. 
These additional requirements are pre- 
sented separately in this section. 

For submicrometer particles, it has 
been found 5 that sufficient constraint 
for pneumatic sampling is that 25% of all 
particles with diameters equal to 
0.015 um be transmitted through the long- 
est sampling tube. For laminar flow in a 
tube (Reynolds number < 1,800), the fol- 
lowing relationship must be satisfied in 
order that this constraint be met:^ 



In order to satisfy both this con- 
straint and the laminar travel time con- 
straint of equation 3, the following re- 
lationship between the length and the 
tube diameter must be met: 



£ n SMP < 640 d, 



'O 



(22) 



where the superscript "SMP" denotes the 
maximum tube length for pneumatic sam- 
pling of submicrometer particles. 

If equation 22 is compared to the maxi- 
mum recommended tube length (eq. 15), 
then it can be seen that the ratio is 
equal to 



4D 



d ^ < 0.30 



(20)* 



*a 



SMP 



156 



(Q™) 



REC 



AP m d 



(23) 



m"o 



For 0.015-u.m particles, the diffusion 
coefficient, Dj , has a value of 3.3 
x 10 -l+ cm 2 /s 7 , so that the tube travel- 
time t£ , must satisfy 



225 d/ 



(21) 



5 Work cited in footnote 4. 

^Fuchs, N. A. The Mechanics of Aero- 
sols. Pergamon Press Ltd./ London, 1964, 
pp. 184, 204-205. 

^Work cited in footnote 6. 



Consequently, when d < 0.519 cm, as- 
suming AP m =580 mm Hg, the maximum tube 
length is limited by the pump capacity 
and when d Q > 0.519 cm, the maximum tube 
length is limited by the required trans- 
mission of smoke particles. In general, 
most rapid pneumatic monitoring systems 
will require standard tubing with >0.635- 
cm (1/4-in) ID; thus, most particle moni- 
toring systems will have their maximum 
tube lengths limited by equation 22. For 
convenience, the maximum tube length as a 



19 






function of inside tube diameter is 
plotted in figure 8, for pneumatic moni- 
toring systems designed specifically 



for measuring 
concentrations. 



submicrometer particle 



SUMMARY AND DISCUSSION 



In general, there exist two rather 
broad applications for monitoring sys- 
tems. One application (category 1) is 
for hazard detection along an entry (or 
entries) for which the point of origin 
of the hazard may not be well defined 
(for example, see Design Example I). In 
this type of application, the spacing of 
sampling points along the entry and 
the resultant contaminant transport 
time between sampling points contribute 
significantly to the overall response 
time of the system. For a pneumatic 
monitoring system with the central 
control station located at the mid- 
point of the entry, the response times 
can be estimated from the following 
equations: 



2.! AetsampN 



1/2 



+ 0.0105 l^'l 
for contaminant gases; or 



(24) 



> 2.1 f &ETSAMP 



1/2 



+ 1.44 x 10 _1 + £ E 2 
for submicrometer particles. 



(25) 



For instance, from Design Example I (£ E 
= 1,800 m, tsamp = 37.5 s, and Vf = 0.76 
m/s), equation 24 would have predicted 
a system response time of ~856 s. The 
final calculated system response time for 
this example was 861 s. 

The second type of application (cat- 
egory 2) is for hazard detection when the 
point of origin of the hazard can be 
reasonably well defined. In this type of 
application, monitoring points are 
located in close proximity to the prob- 
able hazard origin so that contaminant 
transport times can be assumed negligible 



(for example, see 
II). The approximate 
a pneumatic monitoring 
instance, is given by 

«t/3 



Design Example 

response time for 

system, in this 



T S > 0.030 AMAX + nTSAMP (26) 



for contaminant gases; or 

T S > 6.0 x 10"^ £§|AX + nTSAMP (27) 



for submicrometer particles. 

For instance, in Design Example II, 
airway A2 Umax = 2,650 m, n=6, tsamp = 
40 s), equation 26 predicts a response 
time of 1,340 s, while the actual calcu- 
lated value was 1,360 s for the final 
design. 

These four equations can be used in 
making initial estimates of pneumatic 




0.5 1.0 1.5 

INSIDE TUBE DIAMETER,^, cm 



2.0 



FIGURE 8. - Maximum tube lengths as a function 
of inside tube diameter for pneumatic monitoring sys- 
tems designed to detect submicrometer particles. 



20 



monitoring system response times to de- 
termine if this type of system can be 
used in a particular application. In 
general, if the estimated value of i s for 
a proposed application is within ±10% of 
the required response time, x m , or if the 
estimated value of t s is significantly 
less than t m , then a pneumatic monitoring 
system should be more seriously consid- 
ered for the proposed application. 

In formulating a decision regarding 
the potential design and fabrication 
of a pneumatic monitoring system, the 
following initial questions should be 
addressed: 

1. What is the intended function of 
the monitoring system? 

2. What is the maximum hazard response 
time, x m , that can be tolerated in this 
application? 

3. To which applications category does 
this application belong? 

Category 1 — Wide-area coverage for 
which the point of origin of the hazard 
is not well defined; or 

Category 2 — Localized coverage for 
which the hazard origin is reasonably 
well defined. 

4. If the application belongs in cate- 
gory 1 , then for each entry to be moni- 
tored, the entry length and average entry 
ventilation velocity should be specified 



and used in either equation 24 or equa- 
tion 25, depending upon whether the con- 
taminant to be monitored is a gas or sub- 
micrometer particles, respectively. In 
utilizing either equation 24 or 25, an 
average value of t$amp = 45 s will gener- 
ally suffice. 

In some instances, an entry may be too 
long so that a single central control 
station with associated monitoring points 
may not provide sufficient time response. 
In these instances, some thought should 
be given to subdividing the entry, with 
each subdivision having its central con- 
trol station and associated monitoring 
points. 

The example, if a proposed application 
requires that x m < 1,200 s (20 min) and 
the entry is defined by % £ = 4,830 m 
(3 miles) and v f = 1.02 m/s (200 f t/min) , 
then x s (from eq. 24), would have an 
estimated value of ~1,830 s (30.5 min), 
which is considerably longer than the 
required response time of 1,200 s. How- 
ever, if the entry is subdivided into two 
equal lengths (2,415 m each), then two 
central control stations would be re- 
quired with each subsystem having an 
estimated time response of ~1,026 s (17.1 
min) , which is much less than the re- 
quired response time of 1,200 s. 

Once x s nas been verified for a pro- 
posed category 1 application, then the 
number of monitoring points required for 
this application can be estimated from 



(x m -0.012 iE */3) - y<T M -.012 * E «"3)2_ 4 T SAM p|f 

m i n o t 

SAMP 



(28) 



for contaminant gases; or from 



( Tm - 1.6 x 10-4 £ E 2) _ J( Tm - 1.6 x 10-4 £E 2)2 _ 4 



n m i n = 



2x 



SAMP 



SAMP ^~ 



(29) 



for submicrometer particles. 



21 



n must always be an integer, and for 
solutions to equation 28 or equation 29, 
which lie between two integer values, the 
next largest integer value should always 
be used for the initial estimate. For a 
central control station located at the 
midpoint of the entry (or portion of 
entry) to be monitored, equation 28 or 29 
is a valid and reliable estimate for the 
number of monitoring points. However, in 
the final design, it may be possible to 
reduce n through a more judicious selec- 
tion of the central control station 
location. 

For instance, applying equation 28 to 
the application defined in Design Exam- 
ple I U E = 1,800 m, v f = 0.76 m/s , tsamp 
= 37.5 s, x m = 900 s), n min has a value 
of 5.49, or six monitoring points, the 
value obtained in the example when the 
central control station was located at 
the midpoint of the entry. However, in 
the example, it was found that by center- 
ing the central control station with 
respect to the monitoring point, n could 
be reduced from 6 to 5. 

5. If the application belongs in cat- 
egory 2, then a suitable location should 
be chosen for the central control sta- 
tion. In general, this location should 
be central with respect to the monitoring 
points in order to minimize tube lengths 
and improve upon system response times. 
Once the number of monitoring points are 
defined, the distance from the central 
control station to the farthest point 
defines l^^ and either equation 26 or 27 
can be used to estimate the system re- 
sponse time relative to t m . 

Locating the central control station 
centrally with respect to the monitoring 
points can have a very significant effect 
upon the system response time and also 



reduce the size of tubing required. For 
instance, if it had been possible in 
Design Example II to locate the central 
control station more centrally (in the 
center of airway 2, for instance), then 
£ MAX would have been ~1,300 m and the es- 
timated response time of the system (from 
eq. 26) would be ~670 s — almost a factor 
of 2 more rapid in response. Also, 
the size of tubing required for the long- 
est sampling tubes would have been re- 
duced from 1.11 cm (7/16 in) to 0.953 cm 
(3/8 in). 

Again, once t s has been found to be 
acceptable for a proposed category 2 ap- 
plication, then more detailed design 
plans can be initiated. For a category 2 
application, the initial minimum number 
of monitoring points are usually well 
defined, and additional monitoring points 
can be added without seriously affecting 
the original, primary function of the 
system. For instance, in Design Exam- 
ple II, the system's primary function was 
to monitor for excessive methane accumu- 
lation. The addition of three CO moni- 
toring points increased the response 
time of the system by only 120 s, yet it 
now provides for protection from a sec- 
ondary hazard in addition to providing 
more information relative to the methane 
content. 

To summarize, a proposed application 
should be identified as belonging to 
either category 1 or category 2, and the 
appropriate equations used for estimating 
the system response time for the proposed 
application. If the estimated value for 
t s falls within the limits previously 
discussed, relative to some required 
hazard response time, x m > then it is 
reasonable to assume that a pneumatic 
monitoring system can be successfully 
implemented for this application. 



CONCLUSIONS 



The basic components of pneumatic moni- 
toring systems have been described and 
detailed design criteria presented for 
rapidly responding pneumatic monitoring 
systems. The design examples show how 



this information can be used in the 
design of such systems. The information 
contained in this report should be suffi- 
cient for designing pneumatic monitoring 
systems for many potential applications. 



22 

APPENDIX. —LIST OF SYMBOLS 

Dj subraicrometer particle diffusion coefficient, cm 2 /s. 

d m inside diameter of connecting tubing from sample pump to "TEE" connector, 

centimeters. 

d Q inside diameter of a pneumatic sampling tube, centimeters. 

d s inside diameter of tubing connecting the contaminant sensor to the main 

pump exhaust line, centimeters. 

I distance from point of origin of a contaminant to a downstream pneumatic 

sampling point, meters. 

Iq the distance between two consecutive pneumatic sampling parts, meters. 

£ E the length of a mine entry, meters. 

l m the length of tubing connecting the sample pump exhaust to the "TEE" 

connector, centimeters. 

£ MAX the maximum sampling tube length within the pneumatic sampling system, 
meters. 

l the length of a pneumatic sampling tube, meters. 

£, MAX the maximum sampling tube length, in meters, which can be used with a 

fixed sampling tube inside diameter, d Q , and a pump with a maximum rated 
pressure drop, AP m , in mm Hg. 



(£ max )rec tne maximum recommended sampling tube length, in meters, for use with a 
fixed sampling tube inside diameter, d Q , and pump with a rated maximum 

'0 'KtL, _ w.^J k, 



pressure drop, AP m . U MAX )rec = 0.90 I. " 



£ SMP the maximum sampling tube length, in meters, which can be used for 

pneumatic sampling of submicrometer particles through a sampling tube 
of inside diameter, d Q . 

l s the length of tubing connecting the contaminant sensor to the main sample 

pump exhaust line, centimeters. 

n the number of sampling tubes for a pneumatic sampling system. 

n min the estimated minimum number of sampling tubes required to provide a 
system response time equal to, or less than, some required, maximum 
response time. 

P A atmospheric pressure = 760 mm Hg. 

P.J. pressure at the end of a sampling tube prior to entering the sample pump, 
mm Hg. 



23 

AP the difference between P A and P + , (P A -P t ), mm Hg. 

AP m the maximum rated pressure drop for a given pump, mm Hg. 

Q free air capacity of the sample pump, cm 3 /s. 

Q s flow rate required by a contaminant sensor, cm 3 /s. 

Qscav free air capacity of the scavenger, or purge, pump, cm 3 /s. 

Q v required volumetric flow rate, cm 3 /s, through a sampling tube of inside 
diameter, d Q , and length, i Q , 

Vf ventilation air velocity within a mine entry, m/s. 

To the maximum travel time, seconds, for laminar flow through a tube of 
inside diameter, d Q , and length, S, Q . 

T m the maximum anticipated hazard development time, seconds. 

Tp the time required to purge the tubing connecting the sample pump to the 
"TEE" connector, seconds. 

T p the time required to purge the tubing connecting the contaminant sensors 
to the main sample pump exhaust line, seconds. 

tr the time response of the contaminant sensor, seconds. 

t s the maximum calculated response time for a pneumatic monitoring system, 
seconds. 

T SAMP tne sampling time per individual sampling tube within the system, 
seconds. 

t$eq the time required to sequence through all of the system's sampling tubes, 
seconds. 

T+ the time required for the contaminant to travel in the ventilation flow 
from its point of origin to a pneumatic sampling point, seconds. 



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